wAbstract
@This study presents lithium, magnesium and silicon isotope ratios
from pore waters and soils from a well-characterised Histic Andosol
in south-west Iceland. The soil ย7Li composition ranges
between values slightly lighter than basalt, to those that are
much heavier (-1.1๑ to +26.8๑), and are possibly influenced by
sea salt. In contrast, precipitation-corrected dissolved (pore
water) ย7Li values (1.8-10.0๑) appear to reflect preferential
adsorption of 6Li onto secondary minerals, where allophane
supersaturation results in high ย7Li values. Conversely
low ย7Li together with high [Li] are probably due to
destabilisation of allophane at low pH, and thus desorption of
Li. When compared to Icelandic river values, it would appear that
soil pore waters reflect at intermediate isotope composition between
basalts and river waters. Precipitation corrected pore water Mg
isotope ratios (ย26Mg) range between -0.46๑ and -0.12๑,
and correlate with the amount of heavy Mg adsorbed onto the soil
exchange complex. Silicon isotopes in the soils are isotopically
lighter (ย30Si = -0.91๑ to -0.53๑) than basalt (-0.29๑),
whereas pore waters are heavier (+0.13๑ to +1.03๑). Soil ย30Si
values show a clear evolution between unweathered basalt and a
hypothetical isotopically light endmember representing secondary
minerals. Dissolved Si isotopes also respond to chemical weathering
processes, and show that isotopically heavy ย30Si corresponds
to high cation fluxes and high secondary mineral formation. However,
comparison of all these proposed isotopic weathering tracers suggests
that they respond differently to the same chemical weathering
conditions. This indicates a differing behaviour during secondary
mineral neoformation or adsorption depending on whether the incorporated
element is a major or trace constituent. In turn, this behaviour
can potentially yield important information on secondary mineral
behaviour and destabilisation, and thus on the chemical weathering
processes.
Keywords: soils; pore waters; chemical weathering; secondary minerals;
isotope fractionationx
1. Introduction
2. Sample site
3. Methods
4. Results
@4.1. Major element concentrations
@4.2. Lithium and lithium isotope ratios
@4.3. Magnesium and magnesium isotope ratios
@4.4. Silicon and silicon isotope ratios
5. Discussion
@5.1. Elemental ratios
@5.2. Li isotope ratios
@5.3. Mg isotope ratios
@5.4. Si isotope ratios
@5.5. Comparison to river water
@5.6. Isotopic co-behaviour
6. Conclusions
Acknowledgements
References